[Bug fix] Support `:numpad8` for `send_keys` method

[pocke]

:numpad9 is duplicated instead of :numpad8, so send_keys method raises an error when it receives :numpad8. We can reproduce this behaviour with the added test case.

This pull request fixes the problem.

Note

I found this bug with RUBYOPT=-w. For example: (The second warning)

$ ruby -Ilib -rcapybara-webkit -e ''
/path/to/lib/ruby/gems/2.6.0/gems/addressable-2.5.2/lib/addressable/idna/pure.rb:154: warning: assigned but unused variable - startercc
/path/to/lib/capybara/webkit/node.rb:217: warning: duplicated when clause is ignored

[mhoran]

Thanks!